1. Field of the Invention
This invention relates to electronic linear tape measures or rules. More particularly, this invention relates to a low cost and contamination resistant electronic tape measure which digitally displays a length measured by its tape.
2. Description of the Related Art
Electronic tape measures have been known and available for a number of years. In many such devices, the length measured is determined by mechanically or optically tracking the length of a tape unwound from a rotating take-up reel located within a case of the tape measure. In tape measures using incremental systems, electrical pulses are generated as the tape is unwound from the take-up reel. The counted number of pulses correspond to the length of tape unwound from the take-up reel. The pulses are counted and the measured length is displayed on the display.
The counting function may be implemented by an encoder. Such an encoder may be provided on the take-up reel, for example, by mechanical contacts defining multiple rotary switches coupled to the motion of the take-up reel.
These known measuring devices, although capable of providing measurement readings at reasonably precise intervals, require close tolerances and relatively expensive parts to accurately convert the linear motion of the tape to a corresponding rotation of the rotary transducers. Furthermore, these known measuring devices tend to be unreliable and inaccurate when subjected to a mechanical shock. In these known measuring devices, mechanical shock can result in missed or spurious pulses. In a purely incremental system, missed or spurious pulses cannot be corrected.
Using the tape blade to encode absolute displacement data and to employ various optical readers to read the encoded data on the tape blade is also known. However, using photosensors to read absolute displacement data printed on a tape also involves problems, such as providing sufficient light, the high power consumption of the optical readers, mechanical damage to the displacement data on the tape surface, and/or contamination by dirt, grease and the like. Mechanical damage and contamination occur frequently under normal use. Such damage and contamination render these optically encoded devices highly unreliable for the most common uses of these tape measures.
In addition, some of the known electronic measuring devices that employ optical techniques have absolute displacement data imprinted on the tape surface. The absolute displacement data is provided by reflective and non-reflective bar-code elements. These previously known devices use multiple component optical technology and high resolution optical elements for reading the coded tapes. These optical elements require the coded tape to be precisely printed and are complicated to assembly. As a result, these measurement devices are expensive and difficult to manufacture.
Another approach, which combines certain methods of incremental and absolute optical encoding, is described in U.S. Pat. No. 5,027,526 to Crane. Crane uses an optical encoder that provides absolute position readings in combination with an incremental encoder. The incremental encoder provides updated measurements between the absolute position measurements. This absolute encoder corrects for missed or spurious pulses generated by the incremental encoder at certain intervals. The incremental encoder outputs position information frequently enough for the desired resolution of a conventional tape measure. Crane also teaches conserving power by performing the spaced absolute encoder readings using the high-power optical sensor only when a designated pulse is received from the incremental encoder.
U.S. Pat. No. 4,638,250 to Shen-Orr et al. discloses a contactless position sensor with a coreless coil coupling component. The inductive position transducer assembly includes a magnetic field generator and sensor which create and receive a magnetic field coupled to a loop. The displacement of the coupling coil varies the degree of coupling between the transmitter and the receiver such that the magnitude of the induced electrical current indicates the location of the loop relative to the read head.
However, systems such as those described by Crane and Shen-Orr are also subject to mechanical damage and contamination that prevent accurate absolute measurement optical readings. Also, such systems are not truly absolute, since they rely on spaced absolute measurement readings. That is, these systems require a tape displacement sufficient to scan the absolute symbols. Additionally, the high-power optical sensor is a significant power drain on a conventional power source for a hand-held tape measure, even when such sensors are only intermittently used.
An electronic tape measure is needed that: 1) is insensitive to contaminants such as oil and ferromagnetic particles; 2) consumes low amounts of power; 3) is accurate; and 4) is inexpensive to manufacture. An electronic tape measure providing at least these four benefits has until now been unavailable.